Very nice aaa battery pack for Classic Calculators

[teenix]

(06-03-2016 12:53 PM)Dieter Wrote:  Noone can say this for sure unless the electronic specification of the particular calculator is known. But this topic has been discussed here several times over the years. I only remember posts that say Alkalines are fine. After all, if a new Alkaline has 1,55 V and a fully charged NiMH 1,45 V, that's merely 7% difference. But again, I do not think there can be a definitive answer, unless...

Dieter

Okey doke, I used a HP-45 CPU board and ran it through a range of voltages. I got to say, for such a simple switch mode supply it does pretty well.

The voltage range I used was from 3.16V to 4.5V.

VGG stays steady around -12.30V
VSS stays steady around 6.06 - 6.09V

The issue is VCC, it changed from 7.05(3.16V) to 8.3(3.75V - standard) to 9.9V (4.5V) and would go higher for higher battery voltages.

So VCC increases by 1.6V using 3 x 1.5V alkalines - theoretical 4.5V. I don't know if this puts stress on the chips, but it might do because of their age.

The other issue if the peripheral devices that run directly from the battery, coils for the LED displays, sense IC etc. These will have increased voltages but again, I can't say if they will be damaged, and I don't want to destroy a board to find out :-)

The Classic charger supplies a fixed voltage for the CPU board during charge, and the batteries are charged separately so if the charging battery voltage rises it won't affect anything, unless after charging the loaded voltage is above the normal 3.75V.

cheers

Tony